Air lay web forming technology has developed to the point where it is now being used commerically to manufacture a variety of absorbent web products. For example, Scott Paper Company has developed an air lay process in which wood pulp fibers and textile fibers are blended together to manufacture webs for use in impregnated baby wipes, household towels and a variety of industrial wipers. These textile fibers enhance web strength by reinforcing the structure. In addition, Scott owns a number of patents directed to its blended-fiber, air lay technology (e.g., U.S. Pat. Nos. 4,134,948; 4,130,915; 4,097,965 and 4,064,600). This list of patents does not include all patents directed to Scott's air lay technology; however, it is representative of information relevant and material to the subject matter of the instant invention.
Johnson & Johnson also has developed an air lay technology for manufacturing absorbent webs from a blend of wood pulp fibers and textile fibers. This technology purportedly utilizes the suction bonding technique described in U.S. Pat. No. 3,663,348, issued to Liloia et al., to bond the webs throughout their thickness, and purportedly is being commercially utilized to manufacture the facing sheet of Johnson & Johnson's disposable diaper product. This technology also may have been employed at different times to manufacture other absorbent web products.
Karl Kroyer of Denmark has developed an air lay technology employing 100% wood pulp fibers in forming air lay webs. In other words, Kroyer's technology does not employ textile fibers blended with the wood pulp fibers to reinforce the web structure. This technology presently is being utilized by American Can Company to commercially manufacture a household towel under the Bolt trademark, and purportedly employs the method claimed in U.S. Pat. No. 3,669,778, issued to Rasmussen.
In the commercial products employing blended fiber compositions (i.e., wood pulp fibers and textile fiber) relatively large quantities of binder have been needed to establish the necessary strength levels; particularly for absorbent products encountering rigorous use (e.g., household wipers, industrial wipers, etc.). In fact, the prior art technology employed by Scott Paper Company to manufacture its household and industrial wipers relies upon the use of approximately 8.5-10.2 g/m.sup.2 (5-6 lbs. per ream 2,880 ft..sup.2) of a cross-linking latex binder to establish a cross-direction wet tensile strength that does not fall below 0.107 Kg/cm (0.6 lbs. per inch). Although for some applications a lower cross-direction wet tensile strength on the order of about 0.089 Kg/cm (0.5 lbs./inch) can be tolerated, a high binder level in excess of about 6.1 g/m.sup.2 (3.6 lbs./ream) would still be needed.
The need for high binder levels is even more acute in air lay webs formed from 100% wood pulp fibers, since no additional strength will be imparted to these webs by longer-length textile fibers. For example, the commercial use of Kroyer's technology, as is partially reflected in earlier-referenced Rasmussen U.S. Pat. No. 3,669,778, to manufacture Bolt household towels utilizes in excess of 13.6 g/m.sup.2 (8 lbs./ream) of binder to establish a cross-machine direction wet strength level of about 0.135 kg/cm (0.76 lbs./in).
Liloia et al. U.S. Pat. No. 3,663,348, assigned to Johnson & Johnson, suggests the use of low binder levels in air lay web structures. However, in order to achieve desired strength levels Liloia et al. appears to require the use of reinforcing synthetic fibers having a substantially uniform length greater than 1.9 cm (3/4 inches), and a suction bonding technique for bonding the entire web throughout its thickness.
The webs described in the Liloia et al. examples are not embossed, and there is absolutely no teaching that any relationship exists between the magnitude of embossing pressure and strength. However, such a relationship has surprisingly been recognized by applicant, and has been made use of in inventing the products covered herein, as will be explained in greater detail later in this application. It is significant to note that even though specific examples in the Liloia et al. patent suggest the use of low binder levels less than 3.4 g/m.sup.2 (2#/ream), commercial products purportedly covered by this patent employ considerably higher levels above 8.5 g/m.sup.2 (5 lbs. per ream) to obtain a CDWT level of about 0.14 kg/cm (0.8 lb/in).
A further patent of interest relating to air-lay fibrous webs including blends of wood pulp fibers and longer-length fibers, is U.S. Pat. No. 2,926,417, issed to Duvall. The webs disclosed in Duvall are primarily intended for use as flexible insulation and cushioning felts and are not embossed. Duvall definitely does not suggest any relationship between the magnitude of embossing pressure and strength; a relationship that need to be recognized by applicant in making the instant invention. Duvall indicates that to provide sufficient strength in his web to permit it to be conveyed through a bonding operation without falling apart, he needs to include at least 20% textile-length fibers in the structure. Interestingly, the blended fibrous webs of the instant invention can include considerably less than 20% textile fibers and still be carried through a bonding operation in a reliable manner.